An electric double-layer capacitor has a built-in electric storage element impregnated with non-aqueous electrolyte solution (=non-aqueous solvent in which electrolyte salt is dissolved). This non-aqueous electrolyte solution generally uses propylene carbonate or other carbonate ester as a non-aqueous solvent, but because carbonate ester reacts with water and breaks down during electrical charging, non-aqueous electrolyte solutions using sulfolane or other cyclic sulfone having lower reactivity with water than carbonate ester for the non-aqueous solvent have been proposed (refer to Patent Literature 1).
However, cyclic sulfones have higher viscosity than carbonate ester and therefore an electric storage element using a cyclic sulfone of higher viscosity tends to have higher resistance, and for this reason non-aqueous electrolyte solutions using dimethyl sulfone or other chained sulfone having lower viscosity than cyclic sulfones for the non-aqueous solvent have been proposed (refer to Patent Literature 2).
However, chained sulfones, while having lower viscosity than cyclic sulfones, become highly viscous in a low-temperature environment, especially below 0° C. As the viscosity of the chained sulfone in the electric storage element increases, the resistance value of the electric storage element rises beyond the practical range, and consequently the charge/discharge characteristics of the electric double-layer capacitor itself drop significantly.
It should be noted that such electric double-layer capacitors are considered important power sources suitable for backing up the memory, etc., for mobile phones, notebook PCs, video cameras, digital cameras, and other electronic devices. These electronic devices are often used in a low-temperature environment below 0° C., so there are very strong needs for a non-aqueous electrolyte solution for electric double-layer capacitors that does not cause the resistance value of the electric storage element to rise even in a low-temperature environment below 0° C.